The Heckler and Koch 416 assault rifle bears a strong resemblance to the M4 carbine, at least on the outside. Inside it’s a different ball game, as the weapon uses an operating system designed to avoid the pitfalls of the M4-series. The result is perhaps the ultimate M16-type rifle, a gun with all the advantages of a proven, half-century old design and none of the disadvantages.

The original AR-15 rifle was invented in the late 1950s by Eugene Stoner . The AR stood for ArmaLite Rifle, and the ArmaLite company eventually sold its design to Colt’s Manufacturing Company in 1959. After an initial purchase of the AR-15 for the U.S. Air Force, the U.S. Army and Marine Corps both purchased the rifle, now known as the M16, for personnel bound for the Southeast Asia. The M16 lead to the M16A1, which lead to the M16A2 in the 1980s, and the M-4 carbine of today. The AR-15/M16/M4 series of weapons has served continuously in U.S. military service for more than fifty years.

The M16/M4 platform has shed a number of deficiencies over the past 53 years, including a disastrous switch of propellant during the Vietnam War that contributed to a distrust of weapon that lasted for years. One issue the M16 could never get rid of, however, is a key part of the weapon’s operating system that actively fouls the weapon as it is used.

The M16/M4 uses what is called a gas impingement operating system to cycle the weapon. In the gas impingement system the weapon diverts a small amount of propellant gas generated when the gunpowder burns. This gas is diverted at a small port near the tip of the barrel, sent down a stainless steel tube back to the weapon’s receiver, and blown into a small receptacle in the bolt. This sends the bolt, which had been locked forward during firing, backward. As it does so the bolt pulls back the empty shell casing, ejects it, and picks up a fresh round off the top of the inserted magazine. The bolt flies rearward, bounces off the spring-captured buffer, and then flies forward, inserting the new round into the breech.

The gas impingement system is mature and reliable, but there’s one problem: the hot propellant gasses carry with them carbon and other debris and blow them straight into the receiver. This necessitates frequent cleaning of the weapon’s internals.

The Heckler and Koch 416 assault rifle modifies the system, preventing dirty gas from from entering the receiver. Although the 416 still diverts gas, it uses the gas to push a piston which in turn drives the weapon’s internals. Carbon and other debris are ejected from the rifle shortly after, with the result being the 416 runs much cleaner than the M16/M4 series. Although all service weapons require regular cleaning, a HK 416 will have less buildup inside the rifle and run longer without servicing than weapons using gas impingement.

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Other than operating systems, there is little to differentiate the HK416 from the M16/M4 series. The weapons are very similar from the outside, with nearly identical layouts, and functionally and dimensionally the are nearly identical. The 416 is still charged with a charging handle located above the trigger group, the trigger and magazine well are in the same location, and the 416 will even take the same ammunition and magazines. The use of a full length, four position Picatinny rail functionally identical to that on the M16A4 and M4/M4A1 weapons means the same optics, laser pointers and other accessories can be used on the HK416. The manual of arms for the two weapons is identical.

In many ways, the HK416 is the most modern version of the M16/M4 series. The weapon is slightly heavier, with the weight forward, due to the use of a piston instead of a simple gas tube. The piston system also reportedly runs “cooler” than older designs, as the early ejection of hot propellant gas mitigates heat buildup in the receiver.

Today the Heckler and Koch 416 serves in the U.S. Marine Corps where it is paired with a Trijicon ACOG Squad Day Optic to create the M27 Infantry Automatic Rifle (IAR) . The Marines are also fielding a Designated Marksman version, the M38, equipped with the 2.5-8x TS-30A2 Mark 4 MR/T scope . Finally, the Marines plan to equip virtually all frontline riflemen with the HK416. France, Australia, Italy, Germany, the Netherlands, Norway, South Korea and Brazil also use the weapon, although it is generally reserved for special operation forces and only issued army-wide in France.

The HK416 may be the ultimate—and final—version of a weapons platform in continuous service for more than fifty years. The U.S. Army’s development of a new 6.8-millimeter rifle may mark the end of the M4/M16 series of weapons, although details on the new weapon are lacking and it could indeed use the same basic operating system as the 416. If that’s the case, the Army could end up adopting the 416—in some form—setting the stage for another half-century of service.

Kyle Mizokami is a writer based in San Francisco who has appeared in The Diplomat, Foreign Policy, War is Boring and The Daily Beast. In 2009 he cofounded the defense and security blog Japan Security Watch.